To the extent that Foucault's thought is "Heideggarian" it has little to do with "authenticity" or "freedom." To the extent that Foucault has these concepts, they existed more in his personal life than in any of his major writings.

Habermas is ultimately engaged in a rear gaurd defense of Enlightenment rationality. The Nietzschean "revaluation of all values", Heidegger's "history of metaphysics", Derrida's "deconstruction" amd Foucault's "genealogies" and "archeologies" are in Habermas' eyes attacks on the Enlightenment project of rationality. Habermas sees rationality as the basis of science, constitutional government, science, and human rights. Habermas' central argument in the volume "The Philosophical Discourse of Modernity" is that if Enlightenment Rationality is not defended, Western civilization will drift into Fascism as Germany once did under the bad influence of Nietzsche. (The strawman argument is made through veiled insinuation by Habermas.)

Foucault's arguments are that Enlightenment rationality may have freed many from one sort of arbitrary pre-modern tyranny, but has also created "rational" and "universal" means of social control and oppression-- indeed, forms of oppression that we don't recognize as such because they seem so rational to most who consider them even when real people are confined, punished, manipulated, and killed. Habermas seems to view Foucault's genealogies as attacks on the Enlightenment project itself.

What Habermas fails to understand in Foucault's writings is that the dangerous rationality he brings to light is shared by Western democracies and totalitarian societies of both communist and fascist varieties. In fact, Foucault uncovers just how fascism and communism are creatures of Enlightenment rationalism and thus the fascist tendencies in democratic societies as well.

The issue is that Habermas, understandably, doesn't want his own society to go down the road of fascism again, and thus he blindly grasps everything he can label as "the Enlightenment" and tosses every idea he doesn't understand under the rubric of "irrational." It's called "scapegoating."